HDD and external enclosure recommendations?

To get a clear picture of what I'm dealing with, please click THIS link.
Since I'm running out of room—and I really need more space—I've decided to just bite the bullet and buy an external enclosure and an HDD to slide right in there...🙂
What would you guys recommend? (My budget is sitting somewhere around $500 plus change).
It seems to me that buying the HDD and the enclosure separately is the smartest move, right?
Thanks!

http://www.diskont24.com/product.php?productid=57997

As for everything else, just grab a cooling rack somewhere.

Plus, there’s the possibility that I won't even need an extra power supply for it! 🙂